Pressure Profiling: The Key to Perfect Extraction

七月 7, 2017新聞

Specialty coffee is an innovative industry. It seems like there’s a new way to improve our coffee every year—if not every month! Recently, the focus has been on grind evenness and the ability of Mahlkonig’s EK43 grinder to produce a particle size distribution that enables tasty espresso to be made at higher extraction yields.

With all the excitement over grinders, specialty coffee’s previous poster child—pressure profiling—seems to have been forgotten. However, with the recent announcement of a single group La Marzocco Strada, and the availability to the home user of other machines (such as the Vesuvius), all of which feature pressure profiling, it’s worth re-examining this technology.

Spanish Version: Manejo de la Presión: La Clave para una Extracción Perfecta

Ambient and SpressoAmbient & Spresso—The Vesuvius is one of the most recent domestic espresso innovations to incorporate customisable pressure profiles. It’s a beautiful classic e-61 design with a modern twist. Credit: Antonio Nurri

So, What Is Pressure Profiling?

Pressure profiling is when, during extraction, a machine varies the pressure at which it pumps water through the coffee puck. Most coffee machines aren’t able to do this; their pumps can only operate at a single pressure—typically 9 bar—at any one time, although there is sometimes a non-adjustable period at the beginning of the extraction where water is roughly at line pressure (typically 2-4 bar) which is known as pre-infusion.

There are several types of pressure profiling machines available. Some, like the La Marzocco Strada, allow any combination of pressures to be used throughout the extraction. Others, such as the Sanremo Opera, allow pressures to be set in three distinct stages. And some machines, such as the Slayer, vary the flow rate of the water rather than pump pressure. They’re commonly thought of as offering pressure profiling, but it’s closer to flow profiling. In this article, we’re going to be looking strictly at the former two.

Lever machines have a very basic form of fixed pressure profiling, albeit more by accident than design: the initial pull down of the lever lets water into the grouphead at low pressure, and then the spring that forces the lever back to its original position naturally produces a rising then declining pressure profile.

 Victoria Arduino Athena Leva

Victoria Arduino Athena Leva: A masterpiece design of the old world of espresso. The lever shots are mighty tasty. Credit: @dailypressclt

Full-pressure profiling machines, such as La Marzocco’s Strada, take this further by allowing the user to control the pump pressure precisely throughout the entire extraction. In theory, a whole range of profiles are possible, from a slow ramp up and slow ramp down to the exact opposite. In practice, however, a particular profile dominates: a long pre-infusion, followed by a ramp up to full pressure, then a declining profile in the last third of the extraction. If you’ve ever ordered an espresso-based drink in a shop with a pressure profiling machine, it was more than likely produced in this way.

La Marzocco Strada 2

Stunning custom La Marzocco Strada 2 group at the London Coffee Festival. Credit: @nowavecoffee

也可以看看: 5 Barista-Certified Rules for “Dialing In” Your Coffee Grinder

A Typical Profile

With so many different profiles available, why do we typically stick to the same one?

To answer this question, we need to break down the extraction into three main stages.

1. First Contact 

The first stage of any espresso extraction involves water coming into contact with dry coffee grounds. On a very basic machine, this happens with a considerable amount of force. The pump will operate at a pressure many times greater than the pressure to which your car tyres are inflated.

The consequence of the coffee puck being hit with such force is often channeling, where the water finds the line of least resistance in the coffee bed and causes uneven extraction. This is most definitely a bad thing.

pressure profiles compared

Different pressure profiles can have significantly different effects. Credit: Jonathan Prestridge

To combat this, a pressure profiling machine can be set up with an initial low-pressure (usually 3-4 bar) phase, lasting several seconds, before increasing the pressure significantly. This lets the grounds swell and become more adhesive, which mitigates against channeling. It also helps to limit the migration of fines, the very small particles that are an inevitable consequence of a grinder’s bimodal particle distribution. Fines contribute body to a shot, but too much migration of them to the bottom of the basket excessively slows down the flow, causing poor extractions.

2. Increase in Pressure

The middle phase usually involves an increase in pressure until the chosen maximum level is reached, followed by several seconds at full pressure. This is similar to that produced by a machine with fixed pre-infusion, although the ramp up on a full-pressure profiling machine is more gradual—again, to reduce the possibility of channeling.

3. Ramp Down

The final phase is typically a ramp down leading to the end of the extraction. It makes sense to lower the pressure at this point, since the puck density has decreased significantly and most of the extraction has already taken place. It’s interesting to note that the longer the ramp up to full pressure at the beginning of the extraction, the more a ramp down is needed at the end to maintain a consistent flow rate.

This type of profile tends to suit speciality coffee, since it facilitates a full extraction. The long pre-infusion and ramp up stages enable a finer grind to be used than with a straight 9 bar machine. In turn, this makes it easier to achieve optimum extraction yields with the sort of dense light-roast coffees that are often difficult to extract on traditional equipment.

Other Profiles

There are some people using vastly different profiles, one of which is three times World Barista Championship finalist and current UK Barista Champion Maxwell Colonna-Dashwood. He runs a relatively long pre-infusion followed by a straight 6 bar extraction on his Sanremo Opera. Maxwell has found that the lower average pressure over the course of the shot mitigates against channeling and produces excellent extractions when used in conjunction with his shop’s Mahlkonig EK43 grinder and 15g VST baskets.

ustom wooden panel on the Sanremo Opera Beautiful custom wooden panel on the Sanremo Opera at Colonna & Smalls. Credit: Jonathan Prestidge

What’s Next?

There’s scope for experimentation with new profiles, as Maxwell demonstrates. As grinder technology advances and we understand more about espresso extraction, we can hope pressure profiles will evolve to create even tastier coffee.

Sanremo Opera Can’t get enough of the beautiful Sanremo Opera. Such an incredible design. Credit: @framework_coffee

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Written by J. Prestidge and edited by A. Guerra.

Feature Photo Credit: Coffeetopia (@kahvebarmeni)


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